Make blame UI recover gracefully if Identities haven't been built yet for a commit

Summary:
See PHI1014. We may not have Identities if you race the import pipeline, or in some other cases which are more "bug" / "missing migration"-flavored.

Load the commit data so we can fall back to it if we don't have identities.

Test Plan:

• Wiped out all my identities with UPDATE ... SET authorIdentityPHID = NULL WHERE ....
• Before change: blame fataled with Attempting to access attached data on PhabricatorRepositoryCommit (via getCommitData()), but the data is not actually attached..
• After change: blame falls back gracefully.
• Restored identities with bin/repository rebuild-identities, checked blame again.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19958